U.S. Pat. No. 6,466,036 (reference 1) and JP2005-106665A (reference 2) disclose conventional capacitance detecting apparatus.
The capacitance detecting apparatus is incorporated in a system for controlling an opening/closing operation of a door for a vehicle such as an automobile. A detection signal of the capacitance detecting apparatus is employed as a trigger signal for unlocking the vehicle door. Specifically, a control system is set at a door-unlock allowing mode when an ID code is matched between an in-vehicle controller and an operator approaching the vehicle. In this case, when the operator touches an unlock sensor (electrode) housed inside an outside door handle of the vehicle door, the capacitance detecting apparatus detects changes in the capacitance at the electrode and outputs a trigger signal for unlocking the vehicle door. In other words, the capacitance detecting apparatus detects an intention of the operator for unlocking based upon the changes in the capacitance, so that the trigger signal for unlocking is outputted.
A capacitance detecting apparatus is incorporated in a safety device for controlling a distance between a head of an occupant and a headrest of a seat, thereby evading whiplash injury that may be occur upon a vehicle rear impact. The capacitance detecting apparatus can be employed as a distance sensor for detecting a distance between the head of the occupant and the headrest based upon changes in capacitance in response to a distance between an electrode embedded in the headrest and the head of the occupant.
As illustrated in FIG. 18, the capacitance detecting apparatus disclosed in Reference 1, one end of a reference capacitor Cs is connected to a DC power-supply via an oil/off switch S1. The reference capacitor Cs is connected, at the other end, to a variable capacitor Cx and a on/off switch S2. One end of the variable capacitor Cx is grounded, or connected to a free space, via a sensor electrode. Both ends of the reference capacitor Cs are connected to the on/off switch S3. The reference capacitor Cs is connected, at the one end, to a comparator CMP and a control circuit. The comparator CMP selves as a voltage measuring unit for measuring a voltage at the one end of the reference capacitor Cs.
As illustrated in FIG. 19, first of all, the on/off switches S2 and S3 are closed so that the reference capacitor Cs and the variable capacitor Cx are electrically discharged. Next, the on/off switch S1 is closed so that the reference capacitor Cs and the variable capacitor Cx are electrically charged by the DC power-supply. The voltage at the reference capacitor Cs hence increases up to a level of voltage defined by a ratio between a capacitance of the reference capacitor Cs and a capacitance of the variable capacitor Cx. The on/off switch S1 is then opened and the on/off switch S2 is then closed, whereby the other end of the reference capacitor Cs is grounded. The variable capacitor Cx is electrically discharged, and the voltage measuring unit repeatedly measures the voltage of the reference capacitor Cs. The control circuit counts a number of times before the voltage of the reference capacitor Cs reaches a predetermined voltage level. A presence, or an absence, of changes in the variable capacitance Cx is detected based upon increment/decrement of the number of times.
Reference 1 further discloses a capacitance detecting apparatus for detecting a presence, or an absence, of changes in two variable capacitances. In this capacitance detecting apparatus, a reference capacitor Cs is connected to a first sensor electrode at one end and to a second sensor electrode at the other end. The first sensor electrode is connected to a variable capacitor Cx1 of which one end is grounded or connected to a free space. The second sensor electrode is connected to a variable capacitor Cx2 of which one end is grounded or connected to a free space. The one end of the reference capacitor Cs is connected to a DC power-supply via an on/off switch S1 and is grounded via an on/off switch S2. The other end of the reference capacitor Cs is connected to a DC power-supply via an on/off switch S3 and is grounded via an on/off switch S4. In this capacitance detecting apparatus, the on/off switches S1 to S4 are operated and voltages at both ends of the reference capacitor Cs are measured by two voltage measuring units, respectively. As a result, a presence, or an absence, of changes in capacitance at each variable capacitor Cx1, Cx2 is detected.
In the capacitance detecting apparatus disclosed in Reference 2, one end of a reference capacitor Cs, which is connected to an on/off switch S1, is connected to a DC power-supply, the other end of the reference capacitor Cs is connected to one end of a variable capacitor Cx, and the other end of the variable capacitor Cx is grounded. An on/off switch S3 is connected to the one end, and the other end, of the variable capacitor Cx. The capacitance detecting apparatus alternately repeats a second switch operation, by which the on/off switch S2 is switched to a closed state and returned to an open state, and a third switch operation, by which the on/off switch S3 is switched to a closed state and returned to an open state, following a first switch operation, by which the on/off switch S1 is switched to a closed state. The capacitance detecting apparatus detects changes in a value of capacitance of the variable capacitor Cx based upon a number of times of the second switch operation before the voltage of the other end of the reference capacitor Cs reaches a predetermined voltage level.
In each capacitance detecting apparatus disclosed in Reference 1, 2, a voltage of the sensor electrode changes widely so that large radio noise hence generates. Further, a possible disturbance (noise), which is induced in sync with an operation of the on/off switch S2 or S4, may occasionally influence on an operation for detecting capacitance of the variable capacitor Cx.
A need thus exists for a capacitance detecting apparatus, which is not susceptible to the drawback mentioned above.